Improved reflectance and stability of Mo/Si multilayers

Commercial EUV lithographic systems require multilayers; with higher reflectance and better stability then that published to date. This work represents our effort to meet these specifications. Interface-engineered Mo/Si multilayers with 70% reflectance at 13.5 nm wavelength (peak width of 0.545 nm) and 71% at 12.7 run wavelength (peak width of 0.49 nm) were developed. These results were achieved with 50 bilayers. These new multilayers consist of Mo and Si layers separated by thin boron carbide layers. Depositing boron carbide on interfaces leads to reduction in silicide formation of the Mo-on-Si interfaces. Bilayer contraction is reduced by 30% implying that there is less intermixing of Mo and Si to form silicide. As a result the Mo-on-Si interfaces are sharper in interface-engineered multilayers than in standard Mo/Si multilayers. The optimum boron carbide thicknesses have been determined and appear to be different for Mo-on-Si and Si-on-Mo interfaces. The best results were obtained with 0.4 nm thick boron carbide layer for the Mo-on-Si interface and 0.25 nm thick boron carbide layer for the Si-on-Mo interface. Increase in reflectance is consistent with multilayers with sharper and smoother interfaces. A significant improvement in oxidation resistance of EUV multilayers has been achieved with ruthenium terminated Mo/Si multilayers. The best capping layer design consists of a Ru layer separated from the last Si layer by a boron carbide diffusion barrier. This design achieves high reflectance and the best oxidation resistance in a water vapor (i.e. oxidation) environment. Electron beam exposures of 4.5 hours in the presence of 5x10(-7) torr water vapor partial pressure show no measurable reflectance loss and no increase in the oxide thickness of Ru terminated multilayers. Longer exposures in different environments are necessary to test lifetime stability of many years.